A flow rate meter is known to have a housing and a flow rate sensor. The housing defines a bypass path into which a part of a main flow flows. The flow rate sensor is arranged in the bypass path. Patent Document 1 (DE 102008042807 A) discloses a flow rate meter in which the housing has, as an outlet of the bypass path, a through-hole that is provided with an outer wall of the housing. The outer wall of the housing extends in a main flow direction in which the main flow flows in the housing, and the through-hole is open in a direction approximately perpendicular to the main flow direction.
However, according to experiments conducted by the inventors of the present disclosure, when the flow rate meter has the above-described structure, a vortex may occur in the main flow on a downstream side of the outlet of the bypass path since a flow from the outlet may be separated when being joining the main flow. Accordingly, it is required to reduce a separation of the flow flowing from the outlet of the bypass path when being joining the main flow.
According to Patent Document 1, the flow rate meter has a guide wall 136 and a guide plate 142 to restrict a spatial diffusion of the flow that occurs in a separation area located on the downstream side of the outlet in the main flow direction. However, the guide wall 136 and the guide plate 142 do not restrict an occurrence of the separation of the flow, although the guide wall 136 and the guide plate 142 restrict the spatial diffusion of the flow after the separation occurs.